The arrow

the most trusted mark, and a late arrival

Unicode: U+2192 rightwards arrow · Arrows block U+2190–U+21FF
The job: Point. Direct a person, an eye or a quantity along a vector — and, by extension, assert sequence (next) and causation (therefore). No other mark is obeyed so readily by so many people who were never taught it.
Standards: Codified where it does physical work: the Vienna Convention on Road Signs and Signals governs directional arrows on signage across its signatories, and ISO 7000 and IEC 60417 register arrow forms for equipment. Nothing governs the arrow in an interface, which is where it is used most.
Failure modes: One glyph doing three unrelated jobs — spatial direction, temporal succession, logical consequence — and nothing in the form distinguishes them. An arrow on a map is not an arrow on a button. Rendered small, the head is the only informative part, and it is the first thing lost.
Cultural flips: Direction is parasitic on reading order. In right-to-left scripts next points left, so interfaces must mirror their arrows under Arabic and Hebrew and routinely fail to — the arrow that means forward to one reader means back to another. The vertical axis is worse: an upward arrow means north on a map, increase in a chart, upload in a file manager, and approval in a comment thread.

Sources · 4

documented Vienna Convention on Road Signs and Signals, 1968 — the arrow given legal force on the highway.
documented The Unicode Standard, Arrows U+2190–U+21FF and the supplemental arrow blocks — several hundred encoded arrows, which is itself evidence of how many distinct jobs the form is asked to do.
traditional That the abstract directional arrow is a young device, largely spreading through technical and engineering diagrams from the eighteenth century, long after drawn arrows-as-weapons. The claim is standard in the history-of-diagrams literature and rests on absence of earlier examples, which is weaker evidence than a date.
traditional That the arrow is universally understood. Asserted constantly, including by people shipping software; comprehension testing across scripts and literacy levels does not support it as flatly as the assertion requires.